1. It should be easily manipulated and provide ample working time.
2. It should be dimensionally stable and not shrink or change form after it is inserted.
3. It should seal the canal laterally and apically, conform-i ng to conform-its complex conform-internal anatomy.
4. It should not irritate the periapical tissues.
5. It should be impervious to moisture and nonporous.
6. It should be unaffected by tissue fluids and not corrode or oxidize.
7. It should not support bacterial growth.
8. It should be radiopaque and easily discernible on ra-diographs.
9. I t should not discolor tooth structure.
10. It should be sterile.
11. It should be easily removed from the canal if necessary.
corrosion products themselves were cytotoxic, 34 which i mpeded periapical healing.
At the opposite end of the rigidity spectrum are the pastes. Pastes also fulfill many of the criteria listed pre-viously and can easily adapt to the most complex internal anatomy. However, their extreme flowabil-ity can be a negative factor, and overextension or un-derextension is a frequent result of using a paste tech-nique. 35 The movement of the material occurs along the path of least resistance; pressure is required for the material to flow laterally to fill the anatomic vari-ations of the canal. If less pressure is required to flow through the apex than to flow laterally, the mater-ial will flow through the apex. Unfortunately, the op-erator has no way of knowing where the material is
1 04 Color Atlas of Endodontics
FIGURE 7-7 A mandibular left second molar treated with a Sargenti formulation paste. Note the unfilled mesial canals and the extrusion of material into the mandibular canal, which resulted in
paresthesia.
A B
FIGURE 7-8 A, A treated mandibular left second molar exhibiting mesial bone loss. B, On flap re-flection a vertical root fracture was detected.
flowing except by exposing a radiograph. By the time the radiograph develops, retrieval of overextended ma-terial becomes a surgical matter. Additionally, pastes have been associated with the addition of undesirable and toxic chemicals such as paraformaldehyde, which can produce irreversible tissue damage when extended beyond the confines of the root canal system (Figure
7) .36,37
Currently gutta-percha, a semisolid material, is the most widely used and accepted obturating material.38 Chemically, gutta-percha is the transisomer of polyiso-prene, a naturally occurring relative of rubber. In the production of dental obturating cones, approximately 20% gutta-percha is combined with approximately 65% zinc oxide, 10% radiopacifiers, and 5% plasticiz-ers. Clearly, what clinicians refer to as gutta-percha is really a compound composed primarily of other sub-stances. Unlike rubber, gutta-percha cannot be com-pressed by pressure, being less compressible than water,
which is considered incompressible. 3 9 Excessive con-densation pressure cannot cause flow of gutta-percha and does not improve the seal of a root canal fill 4 0 but can fracture roots (Figure 7-8). Gutta-percha can be made to flow if it is modified by either heat or solvents.
Gutta-percha exists in two distinctly different crystalline phases, which Bunn termed "alpha" and "beta" modi-fications. 41 The naturally occurring form is the alpha form, which melts when heated above 65° C. If it is cooled extremely slowly, the alpha form will recrystal-lize. If it is cooled routinely, the beta form recrystallizes, which is the form in which most gutta-percha exists. Al-though the mechanical properties of the two forms are the same, when alpha phase gutta-percha is heated and cooled, it undergoes less shrinkage than the beta form, making it more dimensionally stable for use with ther-moplasticized techniques.
In addition to its ability to conform to canal irregu-larities, gutta-percha exhibits very low toxicity, being
es-Chapter Seven Obturation 1 0 5
FIGURE 7-9 Gutta-percha is relatively inert in periradicular con-nective tissues.
FIGURE 7-11 A .20 series GT file and a .04 gutta-percha cone.
FIGURE 7-10 Nonstandardized (top) and standardized (bot-tom) cones.
FIGURE 7-12 A, Master cone in place with finger spreader.
B, Accessory cone placed in space created by the finger spreader.
C, Accessory cones in place, completing the obturation process.
sentially inert when in contact with the periapical tissues ( Figure 7-9).42,43 Additionally, it is easily removed if post space is needed or if retreatment becomes necessary.
Gutta-percha does not adhere to the canal walls even when thermoplasticized and still requires a sealer to pre-vent leakage. 44,45
Gutta-percha cones are available in two forms: standardized and standardized (Figure 7-10). The non-standardized cones have relatively small diameter tips com-pared with their larger bodies. Their nomenclature refers to these two dimensions-a "fine-medium" cone has a fine tip and a medium body. Standardized cones are designed with an overall 0.02 mm / mm taper, to match the taper of endodontic files. Recently, as files of various tapers have been introduced, different tapers of gutta-percha cones are now being manufactured (Figure 7-11).
TECHNIQUES
Clinicians should be able to use a variety of obturation techniques because each case is unique and may require
modification of routine procedures for an optimal result.
The following are common obturation techniques cur-rently within the standard of care.
Lateral Condensation
Lateral condensation is the most common technique for obturating the root canal space. This technique can be used in most clinical situations and can be modified to facilitate unusual cases. Before performing obturation with lateral condensation, the clinician prepares the root canal in a continuously tapering manner to an endpoint that ideally coincides with the minor constriction,46often referred to as the working length. A standardized point (the "master cone") is selected with a diameter that is consistent with the largest file used in the canal at the working length (Figure 7-12).
The clinician grasps the master cone with forceps at the point where the distance from the forceps to the tip is equal to the working length and inserts it into the canal. If the fit is correct, the point will exhibit "tug-back," or resistance to removal at working length. A
1 06
FIGURE 7-13 An ovoid distal canal of a mandibular molar with a master cone in place.
radiograph is exposed to verify that the point is correctly positioned in the canal. The cone is then removed, coated with sealer, and reinserted.
Nonstandard points are used to obliterate the re-maining space. A spreader is selected that matches the length of the canal and the taper of the points. Finger spreaders provide better tactile sensation and are less likely to induce fractures in the root than the more tra-ditional D-1 1T spreader . 47 Nickel-titanium spreaders provide increased flexibility, reduce stress, and penetrate deeper compared with stainless steel instruments.41,41 The spreader is introduced into the canal to a depth that approaches within 1 mm of the working lengths° and ro-tated to create a space lateral to the master cone for placement of an accessory cone. The process is repeated, with the cones being condensed until the spreader can no longer penetrate the mass. Only light pressure is re-quired because the gutta-percha is not compressible and because as little as 1.5 kg of pressure is capable of frac-turing the root.51 The excess gutta-percha in the chamber is then seared off and lightly vertically condensed with a heated plugger approximately 1 mm below the orifices to the canals or the cementoenamel junction in anterior teeth.